1. Technical Field
The present invention relates to a receiving device, a radio clock, and a receiving method for receiving communication signals containing time information.
2. Related Art
A radio clock which displays accurate time information by receiving time signals transmitted on radio waves is designed to receive standard radio waves in a long wave band transmitted from a ground base station, or time signals in an extremely short wave band transmitted from a GPS (global positioning system) satellite. Also, a radio clock proposed in recent years as in JP-A-2000-321383 receives time signals contained in radio waves in an extremely short wave band modulated by CDMA (code division multiple access) before transmission and obtains accurate time for the purpose of mobile communication. Since waves in the extremely short wave band are relayed by various base stations, plural types of waves can be simultaneously transmitted. Thus, these waves are known as waves receivable in a more preferable condition in various receiving places such as inside of buildings and underground compared with standard waves in a long wave band and waves from the GPS satellite. However, the electric field intensity of the waves containing time signals and transmitted from the base station is high when these waves are received in the vicinity of the base station. In this case, distortion of the receiving signals is caused due to saturation of an amplifier included in the wave clock. A radio clock disclosed in JP-A-5-37409 prevents this distortion and improves receiving capability by disposing an attenuator and a select switch between an antenna and an amplifier of a mobile communication device. According to this radio clock, the attenuator operates when the electric field intensity of the waves is high, and stops when the electric field intensity is low for reception of radio waves from the base station.
According to the mobile communication device shown in the above reference, the attenuator and the select switch are disposed on the first part of the mobile communication device, that is, on the route of the inputted signals. In this case, insertion loss of the select switch is produced, and adverse effects such as decrease in the impedance matching and mixing of noise are caused depending on the layout of the attenuator and select switch and the length of the wires. As a result, receiving quality is lowered. Moreover, the necessity for providing the attenuator appropriate for high frequency inside the mobile communication device increases the manufacturing cost and size of the mobile communication device.